The authors analyzed the extent of right-to-left shunting in patients with migraine, patients with cryptogenic stroke, and controls. Patients with both migraine and stroke had larger shunts than did patients with migraine without stroke (p = 0.038), patients with no migraine with stroke (p = 0.007), and control patients (p < 0.0001). Patients with migraine have overall larger shunts than nonmigraineurs, particularly if they have had a stroke. Right-to-left shunting may be causally related to migraine and to the increased stroke risk of migraine.